Most prior art projection screens are diffuse and scatter incident light evenly in all directions. Imagery projected either from the front or rear of the screen may be seen from all locations in front of the screen. However, observers receive only a small fraction of the incident light energy, and the image brightness is reduced. Achieving a brighter image requires an increase in projector light output or the use of "high-gain" screens. Projector output is limited as a practical matter due to the high complexity and cost of increasing projector output significantly.
Various design approaches have been attempted for high-gain screens. Screens containing arrays of corner reflectors or refraction-matched glass beads provide increased gain from the screen. However, both techniques provide a retroreflecting screen. The majority of the light from the projector is reflected directly back into the projector.
Arrays of simple specular reflectors may be used to direct the light reflected from the projector in the desired direction. Screens consisting of numerous small lenticular elements may also be used. The individual lenticular elements control the light distribution pattern which, in theory, can provide any desired luminous distribution pattern. Fabrication and maintenance of either the specular reflecting array or the lenticular screen is extremely complex and expensive.
The use of one or more holographic optical elements (HOE) to construct a screen provides optimum screen gain within a specific viewing volume. A screen formed using HOEs in this fashion provides a brighter image to the observer within the smaller viewing volume. Outside this viewing volume, a very dim image, or no image at all, will be visible. Increase in the viewing volume will reduce the gain of the screen, defeating the purpose of the high-gain screen.
Where the screen is to be viewed by more than one observer, particularly if the two observers are displaced by any significant distance, the viewing volume must be increased such that both observers may see the image on the screen. Where a HOE element screen is used in an advanced training simulator, an additional difficulty is encountered, incorrect perspective.
The perspective of each observer will vary, depending upon the viewing position, when the focal length of the image presented on the screen is short. This is particularly evident with a close-range scene such as an aircraft landing or taking off or in helicopter nap-of-the-earth flight. The perspective of two viewers sitting spaced apart may differ significantly.
Moreover, simple expansion of the viewing volume on an HOE screen from a single projector will not provide a correct perspective to each of the observers in this arrangement. Current systems obtain correct perspective views by separating each observer's view into a distinct channel and viewing arrangement. Each observer sees the correct perspective view through his own display channel.
Using the example of an aircraft simulator, the disadvantage of this arrangement can be seen. The pilot's window provides the correct perspective for the pilot, and the co-pilot's window provides the correct perspective for the co-pilot. However, the co-pilot cannot look out the pilot's window and vice versa. Therefore, the degree of reality is significantly reduced.
The present invention alleviates the difficulty of separate perspective views while maintaining the maximum gain provided by a limited viewing volume.